I don't remember where I bought my material last year, but I think it was a local construction materials place. A few years ago I used Thor Tarps. Here are a few sources. It pays to shop around. Greenhouse suppliers often have it. You want plastic reinforced with a scrim, which is a mesh of string in between 2 layers of plastic, usually polyethylene, laminated together.

http://www.thortarp.com/specsheets.html http://www.thortarp.com/applications/clear_grnhse.htm look at "DuraShield" , "DuraSkrim", "thoroshield" and "reinforced polyethylene". Thoroshield 550 is 1 example.

http://www.americover.com/6_mil_reinforced_683_prd1.htm http://www.americover.com/6_mil_reinforced_fire_rated_681_prd1.htm

http://buyplasticnow.com/stringreinforcedplastic.aspx

http://www.tarpsplus.com/supheavdutcl.html http://www.esapco.com/ http://www.cleartarps.net/

http://www.shelters-to-go.com/CLEARtarps.shtml#20x40%20clear%20tarp

The fire-rated stuff might be a good idea if one were using it to cover a heated live-aboard vessel during the winter. Some of the fireproof stuff tends to be yellowish in color, however. Costs somewhat more. Ive used the non-fire-rated material, used both 7 mil and 10 mil stuff. 10 mil is really tough but 6 is probably adequate, certainly for 1 season's use. I got 3 seasons out of some heavy duty stuff, in fact I still have pieces that are still good; the limiting factor was not it tearing over time so much as the accumulation of dirt. I use some tape for sealing some areas (such as a door) and tape won't stick to dirt. Last year I used very light stuff that cost less; it was on November through May. It ended up with some small tears, primarily due to laziness on my part in not protecting much against chafe. Areas where the cover is liable to chafe such as stanchion tops should be wrapped in carpet remnant or similar. Lifelines can have foam pipe insulation over them. This stuff cuts easily with sharp scissors, just run the scissors along. Some of this is available made up into "tarps" complete with grommets and some comes plain in big rolls. having it clear keep sthe boat warmer in winter and more pleasant (plenty of light). Hotter as the weather warms up in spring.